mm/page_alloc.c: rework code layout in memmap_init_zone()

This function is getting full of weird tricks to avoid word-wrapping.
Use a goto to eliminate a tab stop then use the new space

Cc: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>